My bog stock FX 5800 would do 500/1050 on air and reached temps of 45-60C with the side of the case off. The original heatsink was expoxy'd onto the integrated heat spreader above the chip. The RAM also got very hot, much more so to the touch than the gpu heatsink IMO, probably because of lesser air flow.

Anyhow I put my chilled glycol / methanol water chiller on yesterday. This uses an old Dangerden Maze gpu cooler with a copper block stuck on it. The inlet temp was <-25C and the outlet temp about -20C.

Using the driver diplay temp measurement the temp was recorded at 0C which I guess means it does not go below 0C. After a 3dmark run it peaked at 3C before quickly dropping back down to 0C again. Nothing major on the overclock .. 13Mhz up at 513, needs more volts I guess.

Anyhow nice temps for a rather cheap water cooling system on what is fiarly well known for being a fairly hot chip.

That's interesting...
So certainly your overclock wasn't limited by heat (!). I agree that it would seem to be asking for more voltage- I wonder if the 5800U has a different voltage regulation circuit as many users have gotten 550MHZ or so with only factory air cooling...
Try using RivaTuner and see if the Core Voltage rises to 6V during 3D operation: Here is an Ultra graphed showing the rise in CoreV from 4 to 6- I wonder does the non-ultra do the same?

In regards to RAM sinks that is a good point, I actually removed my front two because they got in the way of the waterblock, but i had an mobile air con unit blowing over it all ( it's my benhcing rig so I can take liberties this way ), not sure if anything is going to be that good apart from good air cooling blowing over the whole thing.

I think the graph above is 1.4 in Windows going to 1.6v for 3d apps, that would be normal I guess. The gpu volt controller is an ISL component and has two fb pin outs, I think FB1 is the one to go for.

My bog stock FX 5800 would do 500/1050 on air and reached temps of 45-60C with the side of the case off. The original heatsink was expoxy'd onto the integrated heat spreader above the chip. The RAM also got very hot, much more so to the touch than the gpu heatsink IMO, probably because of lesser air flow.

Anyhow I put my chilled glycol / methanol water chiller on yesterday. This uses an old Dangerden Maze gpu cooler with a copper block stuck on it. The inlet temp was <-25C and the outlet temp about -20C.

Using the driver diplay temp measurement the temp was recorded at 0C which I guess means it does not go below 0C. After a 3dmark run it peaked at 3C before quickly dropping back down to 0C again. Nothing major on the overclock .. 13Mhz up at 513, needs more volts I guess.

Anyhow nice temps for a rather cheap water cooling system on what is fiarly well known for being a fairly hot chip.